Electroreductive Cleavage of Propargylic Acetates Through Polarity Inversion of in situ Generated Allenyl Palladium(II) Complexes and, in Part, Unusual Chemical Reduction Induced by Phosphine Ligand

The electroreduction of propargyl acetates in the presence of a catalytic amount of PdCl₂(PPh₃)₂/PPh₃ in DMF containing Et₄NOTs at ambient temperature under a constant current density (10 mA/cm²) afforded the corresponding allenes in 43-84% yields. The time-course of the reaction was monitored by HPLC, indicating that the allenes were formed immediately after the current was applied and that most of the starting materials were consumed when less than the theoretical amount of electricity (2 F/mol), typically 1.2-1.7 F/mol, was reached. Interestingly a considerable amount of phosphine oxide was formed in the electroreduction media. Neither the reductive cleavage of the propargylic acetates nor the formation of phosphine oxide occurred without electricity being applied. The unusual results suggest that chemical reduction with the phosphine ligand would take place, in part, during the course of the electroreduction. A plausible reaction mechanism is proposed involving electroreductive polarity inversion of in situ generated allenyl Pd^II-complex as well as chemical reduction with the triphenylphosphine ligand leading to allenyl anion species.